<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .w{
            width: 800px;
            height: 1400px;
            background-color: pink;
            margin: 0 auto;
        }
        .fixed{
            position: fixed;
            left: 50%;
            margin-left: 400px;
            width: 50px;
            height: 150px;
            background-color: skyblue;
        }
    </style>
</head>
<body>
    <div class = "fixed"></div>
    <div class = "w"></div>
</body>
</html>